Integrated assessment modeling of a zero-emissions global transportation sector

Currently responsible for over one fifth of carbon emissions worldwide, the transportation sector will need to undergo a substantial technological transition to ensure compatibility with global climate goals. Few studies have modeled strategies to achieve zero emissions across all transportation modes, including aviation and shipping, alongside an integrated analysis of feedbacks on other sectors and environmental systems. Here, we use a global integrated assessment model to evaluate deep decarbonization scenarios for the transportation sector consistent with maintaining end-of-century warming below 1.5 °C, considering varied timelines for fossil fuel phase-out and implementation of advanced alternative technologies. We highlight the leading low carbon technologies for each transportation mode, finding that electrification contributes most to decarbonization across the sector. Biofuels and hydrogen are particularly important for aviation and shipping. Our most ambitious scenario eliminates transportation emissions by mid-century, contributing substantially to achieving climate targets but requiring rapid technological shifts with integrated impacts on fuel demands and availability and upstream energy transitions.

Supplementary Figure 3.Total CO2 and GHG emissions and temperature change.(a) Global CO2 emissions, including both fossil fuel and industry sources and land use change, (b) global total greenhouse gas (GHG) emissions, and (c) the resulting global mean temperature increase, in the four primary scenarios in our analysis compared to the scenarios in the IPCC Sixth Assessment Report (AR6) database that are consistent with limiting end-of-century warming to 1.5°C (gray) and that represent a continuation of baseline trajectories (brown).Scenarios from the AR6 database that were generated by GCAM are shown in brighter colors, dark gray for the 1.5°C-consistent scenarios and golden brown for the baseline scenarios.Note that the GCAM scenarios in the AR6 database employed earlier versions of GCAM than the version used in this study, and thus the scenarios may differ in historical periods due to differences in calibration data and other model updates.Global warming potentials used for the non-CO2 greenhouse gases are from the AR5 report.The 50 th percentile FaIRv1.6.2 surface temperature variable is used for global mean temperature increase for the AR6 scenarios.For the scenarios from this study, global mean temperature change reported in the figure comes from Hector, the reduced form climate model linked to GCAM.The "no policy, tech + behavior change" scenario is a comparison scenario that incorporates the economy-wide behavioral and technological changes used in the 1.5°C scenarios (e.g., reduced demand for transportation services, lower population growth), except for the advanced transportation technology assumptions.Thus, the difference between the reference scenario and the "no policy, tech + behavior change" scenario highlights the effect of the non-transportation-technology assumptions on service provision.The differences between the 1.5°C scenarios and the "no policy, tech + behavior change" scenario isolate the effects of the carbon policy and the transportation technology changes.HSR = high-speed rail, pass-km = passengerkilometers, ton-km = ton-kilometers.pathway, it must meet most of its increases in demand in the elasticity adjustment scenarios using biofuels.Elevated biofuel use in aviation reduces its availability for other transportation modes, most notably international shipping, where hydrogen and electricity must then both make up the deficit in biofuel use and provide for the additional service due to the increased demand.These dynamics are most visible in the income elasticity adjustment scenario, but also occur to a lesser extent in the price elasticity adjustment scenario.

Figure 4 .
Transportation direct and indirect emissions.Global direct and indirect CO2 emissions from the transportation sector in the primary scenarios.Supplementary Figure 8.Total transportation service by technology.Global service provided by technology for the full transportation sector, separated into freight transport and passenger transport, across the four scenarios.Note the different y-axis scales and units; units are trillion passenger-kilometers (pass-km) for passenger transport and trillion ton-kilometers (ton-km) for freight transport.Also note that there is a very small amount of coal-based freight service (exclusively freight rail) that is not shown in the figure as it phases out by 2025 and provides only a miniscule contribution to global freight transport in the preceding periods (0.009 trillion ton-km of service in 2005, 0.007 trillion ton-km in 2010, and 0.002 trillion ton-km in 2015 and 2020).BEV = battery electric vehicle, FCEV = fuel cell electric vehicle.Supplementary Figure 18.Transportation service by technology for aviation and shipping, sensitivity scenarios.Global service provided by technology for international and domestic shipping and long-haul and short-haul aviation in the high transportation technology scenario and the sensitivity scenarios.Note the different y-axis scales and units for each mode; units are trillion passenger-kilometers (pass-km) for aviation and trillion tonkilometers (ton-km) for shipping.For aviation, hydrogen technologies employ hydrogen combustion turbines, while for shipping, hydrogen fuel cell electric vessels are modeled.SSP = Shared Socioeconomic Pathway, income elas adj = income elasticity adjustment, price elas adj = price elasticity adjustment, low bio = low bioenergy, BEV = battery electric vehicle, FCEV = fuel cell electric vehicle.Supplementary Figure 19.Transportation service by technology for modes other than aviation and shipping, sensitivity scenarios.Global service provided by technology for freight trucks, freight rail, passenger rail and highspeed rail (HSR), buses, and passenger cars and trucks in the high transportation technology scenario and the sensitivity scenarios.Note the different y-axis scales and units for each mode; units are trillion passengerkilometers (pass-km) for passenger modes and trillion ton-kilometers (ton-km) for freight modes.Also note that there is a very small amount of coal-based freight rail service that is not shown in the figure as it phases out by 2025 and provides only a miniscule contribution to global freight rail transport in the preceding periods (0.009 trillion ton-km of service in 2005, 0.007 trillion ton-km in 2010, and 0.002 trillion ton-km in 2015 and 2020).SSP = Shared Socioeconomic Pathway, income elas adj = income elasticity adjustment, price elas adj = price elasticity adjustment, low bio = low bioenergy, BEV = battery electric vehicle, FCEV = fuel cell electric vehicle.

Table 2 . Cumulative CO2 emissions, 2020 to 2100, from each transportation mode in each scenario
. HSR = high-speed rail.

Table 4 . Break-even carbon prices for electric and hydrogen-based transportation technologies in selected years
. Break-even carbon prices are calculated as the difference in costs (on a per service output basis) between the low carbon technologies and the corresponding standard refined liquids-based technology, divided by their difference in emissions per service output.We only consider direct emissions here; thus, electric and hydrogen-based technologies are assumed to generate zero emissions.Cost values are used from the reference scenario.Results are shown for the US as a representative example, as costs vary regionally, and include values for shipping and aviation as well as for the rail and road modes that are employed in all regions.A 0 value indicates the technology is already cost-competitive without an additional carbon price.BEV = battery electric vehicle, FCEV = fuel cell electric vehicle.

Table 7 . Cumulative CO2 emissions, 2020 to 2100, from each sector in each scenario
. LULUCF = land use, land use change, and forestry.

Table 8 . Cumulative CO2 emissions, 2020 to 2047 (the year in which net zero CO2 emissions are achieved in the medium and high scenarios), from each sector in each scenario.
The low scenario achieves net zero CO2 emissions in 2048.LULUCF = land use, land use change, and forestry.

Table 9 . Aviation and shipping consumption of FT biofuels and
e-fuels.Fraction of all Fischer-Tropsch (FT) biofuels and e-fuels produced that are consumed by aviation and shipping, in selected years.